Repair of structural members

ABSTRACT

A method of increasing the structural strength of a load bearing member such as a pile ( 12 ), particularly for the purpose of repairing a damaged pile, includes the steps of securing a rigid, structural sleeve ( 17 ) around the pile ( 12 ) and spaced therefrom to form an interspace ( 18 ), filling the interspace ( 18 ) with an expansive filler ( 19 ), and allowing the filler ( 19 ) to set and expand in the interspace ( 18 ) to a prestress of at least 3 MPa and impose tensile hoop stresses in the sleeve ( 17 ) and corresponding compressive hoop stresses in the pile ( 12 ) to enhance the load transfer between the pile ( 12 ) and the sleeve ( 17 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to the repair of structural members and relates particularly to improved methods and apparatus for effecting repairs of structures such as piles in situ.

[0003] The invention will be described with particular reference to effecting repairs of structures such as piles, but it will be understood that the invention has wider application to maintenance, protection and repair of a wide range of structural members including steel jacket offshore structures, structural members on bridges or the like, tubular and non-tubular posts of all types, columns, poles and pillars.

[0004] The invention will also be described particularly with reference to cylindrical metal piles although it will be understood that the invention is applicable to other forms of structures, including concrete piles, timber piles and the like.

[0005] 2. Description of the Prior Art

[0006] Piles, such as those used to support jetties, wharves, piers, offshore structures such as steel jacket platforms and similar structures are subject to damage caused by erosion, corrosion and related chemical or electro-chemical erosion, and by impact such as from marine vessels and the like. Piles which are immersed in water, particularly sea water, are generally subject to most corrosion on that area of the pile between tide levels. That portion of the pile is periodically immersed in water and then exposed to atmosphere causing accelerated corrosion as compared to other parts of the pile. As the structural integrity of the jetty, wharf, pier, bridge, offshore structure or the like can be substantially impaired by damage to the piles, it is necessary to repair or replace any piles which suffer substantial damage.

[0007] Heretofore, the repair of hollow, steel tubular piles has been effected by welding full or part sleeves to the pile so that the sleeves overlie the affected damaged portion.

[0008] A full sleeve may be formed by welding two half sleeves to the pile along longitudinal and end edges thereof thus effectively adding an outer skin to the damaged pile. However, as most damage to piles occurs in the area between maximum and minimum water levels, and as the repair sleeve or part sleeve must extend substantially beyond the damaged area to provide the necessary strength to effect a proper repair, it is generally necessary to effect the welding of such sleeves in the tidal zone or underwater. Such welding is relatively expensive and time consuming, may require special equipment, and 1-0 the welds so made, unless carried out with great skill, can be less than perfect, thus, compromising the structural integrity of the repaired pile.

[0009] Australian Patent Specification No 59285/86 discloses a method of repairing a concrete pile by using a steel sleeve and filling the space between the sleeve with a grout. The method includes removing damaged concrete from the pile before placing the sleeve. The specification discloses the method in relation to a square pile and the sleeve is provided with internal, integral metal spacers formed as part of the sleeve and which bear on the surfaces of the pile.

[0010] Such sleeves are relatively expensive and need to be custom made for each different type, shape and dimension of pile.

[0011] U.S. Pat. No. 4,306,821 discloses a method for reconditioning structural piles whereby a building filler is placed into a space between a flexible sheet form and the damaged piling. The filler provides a protective and structural coating to the damaged piling. With this method, the strength of the repaired pile is dependent solely on the strength of the filler material, which is preferably an epoxy material and on the adhesive interface with the pile. Therefore, the amount of filler required is that amount necessary to provide the desired strength to the pile.

[0012] U.S. Pat. No. 4,697,649 discloses a method of reinforcing a damaged post in the ground by providing a split casing having helical threads at one end which are driven into the ground by a casing rotary drive mechanism. An annulus between the casing and the post is filled with a suitable filler material. However, this system is designed for above ground posts and not for underwater piles. Further, the driving mechanism is relatively complex and expensive and adds substantially to the repair costs.

[0013] U.S. Pat. No. 4,439,070 relates to a method of restoring an under waterpile which uses a flexible jacket placed around the pile and filling the formed annular space between the pile and the jacket with an appropriate epoxy resin compound. Upper and lower seals are provided between the jacket and the pile and the jacket has inlet ports for injection of the epoxy resin and outlet ports for venting water from the annular space. Again, the strength of the repaired pile is dependent on the strength and amount of epoxy resin and on the adhesive interface with the pile, and the system relies on the effective operation of the seals at each end to form the closed annular space. A similar system is shown in Swiss Patent No 573520.

DESCRIPTION OF THE INVENTION

[0014] It is therefore desirable to provide an improved method and apparatus which enables a repair of a damaged pile to be efficiently and effectively carried out.

[0015] It is also desirable to provide a method for effecting the repair of a damaged pile which obviates the need for tidal zone or underwater welding.

[0016] It is also desirable to provide a method and apparatus for carrying out the repair of a damaged pile which can be used in a wide variety of applications and in respect of a wide variety of piles, columns, posts and the like as referred to above.

[0017] It is also desirable to provide a method and apparatus for repairing piles which is relatively inexpensive and which enables the repair to be carried out relatively quickly and economically.

[0018] It is also desirable to provide an improved method and apparatus for carrying out the repair of piles which can be used by relatively unskilled personnel.

SUMMARY OF THE INVENTION

[0019] In accordance with one aspect of the invention there is provided a method of increasing the structural strength of a load bearing member, such as a pile, including, inter alia, the steps of fitting a rigid, structural sleeve to the member, the sleeve having internal dimensions greater than the external dimensions of the member to provide a continuous interspace from the member to the sleeve, supporting the sleeve so that it is spaced from the member, sealing the continuous interspace, filling the space between the sleeve and the member with an expansive filler, allowing the filler to set, cure and expand within the confines of the continuous interspace to generate an expansive pressure of at least 3 MPa which imparts at least radial prestress forces on the member and tensile hoop stresses in the sleeve, whereby the generated forces effect a transition of loading forces from one section to another of the load bearing member through the sleeve and expansive filler.

[0020] The sleeve may be a single piece sleeve for those applications where a single piece sleeve is able to be engaged with the member. Otherwise, the sleeve may be formed of two or more parts which are interconnected, as by welding, bolts or other mechanical connectors, by clipping, by adhesion or chemical bonding, by the use of hinge means or by a combination of these.

[0021] The sleeve is spaced from the member by the use of spacer means, which may include spacing tabs, indentations or other deformations of the sleeve material or separate spacers including spacers formed of synthetic plastic or rubber materials or the like attached to the inner surfaces of the sleeve. Alternatively, spacer means may be attached to the member prior to engagement of the sleeve thereon or spacer means may be inserted between the member and the sleeve after the engagement of the sleeve on the member.

[0022] In a further aspect of the invention there is provided, an apparatus for increasing the structural strength of a load bearing member including a rigid, structural sleeve formed of one, two or more elongate sections to be fastened about a load bearing member, a plurality of spacers adapted to space the sleeve from the member to form a continuous interspace and a set or cured expansive filler prestressed to at least 3 MPa to fill the continuous interspace which generates and imparts at least radial prestress on the member an sleeve such as to set up compressive hoop stresses in the member and tensile hoop stresses in the sleeve, the filler forming a rigid interface between the member and the sleeve.

[0023] In the above form of the invention, a seal is provided between the lower edge of the sleeve and the member to seal the continuous interspace between the sleeve and the member at the lower end thereof. A second seal may also be provided spaced from the first seal, and preferably at the upper edge of the sleeve. The seal or seals act to retain the expansive filler within the space between the sleeve and the member before the filler has set and expanded. When the sleeve is composed of two or more parts, seals may also be used between the parts of the sleeve, depending on the means of fixing the sleeve parts together.

[0024] The expansive filler may comprise a grout, such as a cementitious grout to which an expansive agent is added so that the grout sets and expands on curing. For a cementitious grout, the expansive filler may be a calcium-sulpho-aluminate or lime based mixture. The applicant has found that after the expansive filler is applied to the continuous interspace between the member and the sleeve it begins to cure. The expansive agent used to cause the grout to increase in volume begins to react within the grout. This causes a build up of pressure within the annulus and hence creates a prestress between the structure under repair and the outer sleeve. This pressure or prestress creates an outward force or tensile hoop stresses on the sleeve and an inward or compressive force on the structure under repair. These forces cause extra frictional forces that allow for increased overall capacity of the joint.

[0025] Due to losses in prestress through creep and shrinkage over time, the applicant has found an initial minimum level of prestress of 3 MPa is necessary to ensure that the beneficial effects of the application of prestress according to the invention are not negated. This ensures that the generated forces are imparted throughout the lifetime of the sleeve applied to the load bearing member. Depending on the expansive material used, a prestress of 12 Mpa or more is achievable.

[0026] In a further aspect of the invention, there is provided, an apparatus for increasing the structural strength of a load bearing member including a rigid, structural sleeve formed on one, two or more elongate sections to be fastened about a load bearing member, a plurality of spacers adapted to space the sleeve from the member to form a continuous interspace and a set or cured expansive filler prestressed to at least 3 MPa to fill the continuous interspace which generates and imparts at least radial prestress on the member and sleeve such as to set up compressive hoop stresses in the member and tensile hoop stresses in the sleeve, the filler forming a rigid interface between the member and the sleeve.

[0027] In one form of the invention, shear keys are provided to positively and rigidly connect the sleeve with the expansive filler and to connect the filler with the member. Where the member is a timber pile, shear keys may be engaged with the pile prior to the engagement of the sleeve. The shear keys may comprise studs, bolts, including coach bolts or the like fastened to the member. The sleeve itself may also have inwardly extending shear keys formed by any suitable means including bolts, punched or pressed deformations of the sleeve, welded pins or the like. The shear keys may be preformed so as to also constitute the or some of the spacer means.

[0028] According to another aspect of the invention there is provided a method of repairing a pile having a damage area including the steps of:

[0029] (a) securing a sleeve which has inner dimensions greater than outer dimensions of the pile around the pile;

[0030] the sleeve having an axial length and being secured so as to extend beyond the area of damage in both directions;

[0031] (b) providing spacers to space the sleeve from the pile to form a continuous interspace from the pile to the sleeve;

[0032] (c) sealing a lower end of the continuous interspace;

[0033] (d) introducing an expansive filler through the upper end of the continuous interspace, displacing any water or air in the continuous interspace, to thereby fill the interspace with the expansive filler; and

[0034] (e) permitting the filler to set or cure and to expand within the confines of the continuous interspace to generate an expansive pressure of at least 3 MPa and imparts at least radial prestress forces which tension the sleeve and apply compressive forces to the pile and produce compressive hoop stresses in the pile and tensile hoop stresses in the sleeve, whereby the filler facilitates load transfer primarily by friction across the filler-to-pile and filler-to-sleeve interfaces.

[0035] In order that the invention may be more readily understood, embodiments thereof will now be described with reference to the accompanying drawings wherein.

[0036]FIG. 1 is a part sectional view of a jetty pile and an encasing repair sleeve in accordance with the present invention;

[0037]FIG. 2 is a cross-sectional plan view of a cylindrical, tubular pile and an encasing sleeve in accordance with one embodiment of the invention;

[0038]FIG. 3 is a part cross-sectional elevational view of a lower end of an encasing sleeve in accordance with a further embodiment of the invention;

[0039]FIG. 4 is a schematic, perspective view of a further form of sleeve of the invention;

[0040]FIG. 5 is a view of a part sleeve illustrating a further embodiment of a sleeve seal in accordance with the invention; and

[0041]FIG. 6 is a cross-sectional plan view of a timber pile and encasing sleeve in accordance with another embodiment of the invention.

[0042] Referring to the drawings, the embodiments illustrated in FIGS. 1 to 5 are designed particularly for the repair of a cylindrical, tubular pile 12 such as those used for jetties and the like. FIG. 1 illustrates a jetty superstructure 14 supported by a pile 12 in a sea water environment. The sea water 15 has, over a period of time, caused corrosive damage 16 to the pile 12 over an area on the pile 12 generally corresponding to the tidal limits.

[0043] In order to effect a proper repair and reinstate the integrity of the pile 12, and to provide protection against corrosion, it is necessary to secure a sleeve 17 to the pile 12 so that the sleeve can carry at least part of the load on the pile 12. In accordance with the present invention, the repair is effected by locating the sleeve 17 about the pile 12 and spacing the sleeve 17 from the pile to form a substantially annular space 18. The space 18 is then filled with an expansive grout 19 which expands when it sets thereby tensioning the sleeve 17 and forming a substantially firm or rigid interface between the pile 12 and the sleeve 17.

[0044] In order that the grout is retained in the annular space 18 before setting, a seal 21 is provided at least around the lower edge of the sleeve 17 to extend between the sleeve and the pile 12. The seal shown in FIG. 1 is formed by a flexible, deformable rubber ring separated at a location to allow the separated 1.0 ends to be spaced to engage the ring or the pile. However, the seal 21 may be formed by any suitable means or material, including synthetic plastic material, mastic or the like.

[0045] Preferably, a second seal 22 is provided at the upper end of the annular space 18 to maintain the grout within that annular space during the initial setting and expanding period. Such a seal 22 is fitted in place after the grout has been conveyed into the annular space 18 and all water and air expelled, otherwise, provision needs to be made to allow the water and air to escape.

[0046] It will be appreciated that the sleeve 17, which may be formed of two or more parts, is, when assembled, a load bearing member which is able to carry at least part of the load on the pile. Further, the expansive grout must also be of a material which, on setting, is able to apply a prestress of at least 3 MPa and facilitates load transfer between the pile and the sleeve. The preferred grout for use in this invention is a cementitious grout mixture having as an expansive agent a calcium-sulpho-aluminate formulation.

[0047] Such a formulation has an initial setting period during which the grout hardens. The initial setting period is followed by a curing period during which the grout expands and generates radial prestress forces of at least 3 MPa resulting in tensile hoop stresses in the sleeve 17 and compressive hoop stresses in the pile 12. After approximately 28 days from the time of application of the grout the level of prestress that builds between the sleeve and the repaired structure reaches a maximum value. Grout that experiences constant forces often suffers from loss of strength over time. There are several reasons for the loss of grout strength over time, they include shrinkage of the grout and thus relaxation in any prestress that may exist, creep of the grout due to the applied load and other factors that may occur during the life of the structure.

[0048] If the grout is allowed to cure in the air, the water within the grout will be lost and the size of the grout will be reduced. In the case of a grouted connection, the losses due to shrinkage will be lower than other fully exposed grouted structures. By applying an expansion agent to the grout the present invention overcomes this shrinkage problem.

[0049] With the constant application of load to the grout some increase in strain, hence movement in the grout, or relaxation in prestress may be expected. Depending on the type of grout and the way in which the load is applied the phenomenon of creep can manifest itself in many different ways. By allowing for a high enough level of prestress the present invention overcomes any possible losses due to creep and ensures that more than a negligable level of prestress remains throughout the life of the repair.

[0050] Further, by roughening the inner surface of the sleeve 17, a greater degree of resistance to slip-between the grout and the sleeve is achieved. The surface roughness may be increased by any suitable means including machining, stamping, shot blasting or the like.

[0051] Referring to FIG. 2, a sleeve 17 is illustrated formed of two parts 17 a and 17 b which are held together with two axially extending rows of bolts 24 which engage threaded holes 26 or captive nuts in the opposed sleeve half. The sleeve 17 is assembled around the pile 12 and is spaced therefrom by a series of internal deformable rubber spacers 27 which are fixed to the inside surface of each sleeve part 17 a and 17 b, such as by adhesive or mechanical fastening. The spacers 27 are provided at several locations along the length of the sleeve parts whereby, when the sleeve parts are connected as illustrated, the spacers act to centre the sleeve 17 on the pile 12 so that a substantially annular space 18 exists between the sleeve 17 and the pile 12. This space is then filled with the expansive grout mixture aforesaid which, on setting, expands and produces tensile stresses in the sleeve 17 and compressive stresses in the pile 12, the grout being of a material which facilitates load transfer between the pile and the sleeve.

[0052] Referring to FIG. 3, there is illustrated an alternate form of spacer 28 which comprises a part cylindrical ring 28 formed of a flexible material, such as a spring steel or the like, and which is able to be clipped around the pile 12. The ring 28 has a number of depending, inwardly extending legs 29 the inner edges of which are adapted to engage the surface of the pile so that the body of the ring 28 is spaced therefrom. The ring 28 engages over the lower end of the sleeve 17 thereby supporting that end such that the sleeve 17 is evenly spaced from the pile 12.

[0053]FIG. 3 also illustrates a lower seal 21 formed of one or more pockets 31 of a flexible material, such as rubber, synthetic plastic material, fabric or the like. The pockets 31 are open at their upper end and are secured to the lower edge portion of the sleeve 17 by bonding or by any other suitable fastening means.

[0054] In use, when the expansive grout 19 is introduced into the annular space 18 from the upper end, the grout fills the pockets 31 causing the inner face thereof 32 to engage and seal against the outer surface of the pile 12. The grouted pockets 31 thus form a seal preventing the expansive grout 1.9 passing downwardly and out of the annular space 18. It will be understood that when the sleeve 17 is formed of two or more interconnected part circular parts such as is shown in FIG. 2, the pockets 31 are provided on each of the individual sleeve parts and end pockets abut each other when the sleeve parts are interconnected.

[0055] Referring to FIG. 4, a modified form of sleeve 17 is illustrated. The sleeve 17 is formed of two parts 17 a and 17 b connected together with a continuous hinge 33 which is fixed to the sleeve parts by rivets 34 or welding or other suitable fastening means. The sleeve 17 of this embodiment is able to engage over the pile 12 by opening the sleeve parts about the hinge axis and then closing and securing the parts by means of fastening bolts 24. Appropriate spacers 27 are provided to space the sleeve 17 from the pile 12 and a seal 21 is located at the lower end of the annular space 18 to enable that space to be filled with an expansive filler material, preferably the aforesaid cementitious grout mixture containing an expansive agent which causes the grout to expand on setting.

[0056] Referring to FIG. 5 there is illustrated a further embodiment of the invention in which the sleeve part 17 a is provided with an active seal 21 at its lower end which comprises a bag structure or bladder 36 of substantially toroidal shape attached to the inner surface of the sleeve part 17 a by bonding with adhesive, or by fastening means 37. The bag structure or bladder 36 is flexible and is therefore able to conform to the surface of the pile 12 to which the sleeve part 17 a is to be fitted.

[0057] A grout line 38 extends from an upper end of the sleeve part 17 a to the bag structure 36 and through which the bag 36 is adapted to be filled with grout. In use, the sleeve 17 a is connected with a corresponding sleeve (not shown) about a pile 12, the two parts being secured together by bolts passing through the holes 25. Grout is introduced into the opening 39 of the grout line 38 and fills the bag structure 36 thus forming a close fitting seal against the pile. Adjacent ends of the bag structures of the co-operating sleeve parts abut when the sleeve parts are assembled and the respective bag structures 36 are filled with grout material.

[0058] It will be appreciated that any suitable seal may be provided at the lower end of the annular space 18 to support the expansive grout within that space until such time as the grout has set. Such a seal may be formed of any suitable material, including synthetic plastic material, foamed plastics material, or even welding. Similarly, the upper end of the annular interspace 18 may be sealed 1.0 by welding (in the case of a steel pile 12) or other sealing means.

[0059] It will also be appreciated that the features of the invention can be applied to piles or other structures of non-circular cross-section. In this case, the shape of the sleeve 17 may, if desired, be made to correspond to the shape of the structural member and to provide a space there between into which an expansive filler material can be introduced.

[0060] Referring to FIG. 6, the sleeve 17 illustrated is used to repair a damaged timber pile 12. In this embodiment, shear keys 42 are embedded in the pile 12 and extend into the substantially annular interspace 18. If desired, the sleeve parts 17 a and 17 b may also be formed with internally extending shear key formations 43. It will be understood that such shear keys 42 and 43 may be used in the embodiments of FIGS. 1 to 5.

[0061] The sleeve 17 is spaced from the pile 12 by deformable spacers 27, and a seal, similar to that shown in FIG. 5, is used to seal the lower end of the interspace.

[0062] The filler material 19 is introduced into the interspace 18 through the upper end thereof. The filler may be introduced through one or more tubes or pipes 44 which can extend to the lower end of the interspace to ensure delivery of the filler 19 to that lower end. Any water or air in the interspace 18 is displaced by the filler 19 as it is delivered thereto. When filled, the interspace may be sealed at its upper end to retain the filler during setting or curing.

[0063] The filler, on curing or setting over a period of time such as between five (5) minutes and twelve (12) hours, generates tensile hoop stress in the sleeve and corresponding compressive stress in the member. These stresses enhance the load transfer between the member and the sleeve which is primarily effected by friction across the interfaces.

EXAMPLE 1

[0064] Due to the uncertainty in the expected behaviour of the grout within the sleeve the best method for prediction of the possible losses in prestress is from observation of experimental results. From recent research by Yee Teck Lee, (Yee Teck Lee, Masters Thesis, “Grouted Sleeve Connections of Circular Hollow Steel Members Under Large Deformation Cyclic Loading,” Monash University, 2002), the loss of prestress in grouted sleeves occur in the first 10 to 20 weeks. Further losses appear to be insignificant beyond this time. The average loss of prestress observed in Yee Teck Lee's research was approximately 2.6 MPa, this confirms that a prestress greater than 3 MPa would allow for this expected loss.

EXAMPLE 2

[0065] A method for calculating the capacity of a stressed grouted connection is given in the UK Department of Energy, Offshore Technology Report on Grouted and Mechanical Strengthening and Repair of Tubular Steel Offshore Structures. (R G Harwood, E P Shuttleworth, Department of Energy, “Grouted And Mechanical Strengthening and Repair of Tubular Steel Offshore Strucutres,” Offshore Technical Report, 1988) These equations have been applied (see below) to determine the strength of the present invention for several different configurations. It can be seen from these calculations that if the initial prestress is 3 MPa and this prestress reduces to 0.4 MPa over the joints lifetime the capacity of the section is twice the capacity of the same section without the application of prestress.

[0066] The capacity of the present invention is from the bond between the grout and the sleeve and the friction caused by the application of the prestress. If there were no prestress applied, or if it was at a level such that it may deteriorate to no prestress, the capacity of the joint would be from the bond alone, there would be no contribution from the extra frictional forces from the expansive grout.

[0067] As can be seen in the calculations the capacity of the joint is also dependent on the length of the sleeve. The longer the sleeve the larger the area of contact with the area under repair. If the length of the sleeve is reduced it is necessary to increase the contact force in some way so as to maintain the capacity. By increasing the prestress the contact force increases, this increases the capacity of the joint, therefore the length of the sleeve required can be reduced to maintain a similar capacity, thus decreasing the cost of the repair.

[0068] The capacity of the present invention is greater than that for other such repairs as the prestress developed in the sleeve allows for significant increases in the overall capacity of the connection.

[0069] It will be understood that features described in relation to any one of the described embodiments may be used in the other of the embodiments.

[0070] by Martin Hewitt

[0071] Sep. 24, 2002.

[0072] from “Grouted and Mechanical Strengthening of Tubular Steel Offhsore Structures”

[0073] Offshore Technology Report (OTR) OTH 88 283 from Department of Energy section 1.8.7 pg 56

EXAMPLE 3 (a)

[0074] Static Strength of Stressed Grouted Connections:

[0075] Assess Static Strength with no prestress. Resistance provided by surface friction $\begin{matrix} {{P\quad p}:={0.22 \cdot \left( {\frac{F\quad {n \cdot C}\quad s^{\prime}}{\Gamma\mu} + \frac{{1.75 \cdot 10^{- 3} \cdot A \cdot C}\quad s}{\Gamma \quad b}} \right) \cdot {\quad\left\lbrack {1 + {33 \cdot \left( \frac{D}{T} \right)^{- 1}}} \right\rbrack}}} & {{{Eqn}.\quad 1.8}{.3}\quad {OTR}\quad {report}} \end{matrix}$

EXAMPLE 3(b)

[0076] Assess Static Strength with 1 MPa prestress. Assume 3 MPa with 2.6 MPa loss over 1 year see Yee Teck Lee “Grouted Sleeve Connection of Circular Hollow Steel Members Under Large Deformation Cyclic Loading”. $\begin{matrix} {{P\quad p}:={0.22 \cdot \left( {\frac{F\quad {n \cdot C}\quad s^{\prime}}{\Gamma\mu} + \frac{{1.75 \cdot 10^{- 3} \cdot A \cdot C}\quad s}{\Gamma \quad b}} \right) \cdot {\quad\left\lbrack {1 + {33 \cdot \left( \frac{D}{T} \right)^{- 1}}} \right\rbrack}}} & {{{Eqn}.\quad 1.8}{.3}\quad {OTR}\quad {report}} \end{matrix}$

[0077] Therefore application of a prestress of 3 MPa with a subsequent 2.6 MPa loss of prestress results in a doubling of the capacity.

EXAMPLE 4(a)

[0078] Assess Static Strength of the present invention grouted connection with 10 MPa Prestress and length of 1 D $\begin{matrix} {{P\quad p}:={0.22 \cdot \left( {\frac{F\quad {n \cdot C}\quad s^{\prime}}{\Gamma\mu} + \frac{{1.75 \cdot 10^{- 3} \cdot A \cdot C}\quad s}{\Gamma \quad b}} \right) \cdot {\quad\left\lbrack {1 + {33 \cdot \left( \frac{D}{T} \right)^{- 1}}} \right\rbrack}}} & {{{Eqn}.\quad 1.8}{.3}\quad {OTR}\quad {report}} \end{matrix}$

EXAMPLE 4(b)

[0079] Assess required Length to create similar capacity with no prestress. $\begin{matrix} {{P\quad p}:={0.22 \cdot \left( {\frac{F\quad {n \cdot C}\quad s^{\prime}}{\Gamma\mu} + \frac{{1.75 \cdot 10^{- 3} \cdot A \cdot C}\quad s}{\Gamma \quad b}} \right) \cdot {\quad\left\lbrack {1 + {33 \cdot \left( \frac{D}{T} \right)^{- 1}}} \right\rbrack}}} & {{{Eqn}.\quad 1.8}{.3}\quad {OTR}\quad {report}} \end{matrix}$

[0080] Therefore for an equivalent permissible load, the required length of the sleeve using the present invention is approximately {fraction (1/26)}^(th) of that required for a grouted repair with no prestress 

The claims defining the invention are as follows:
 1. A method of increasing the structural strength of a load bearing member including the steps of: (a) fitting a rigid, structural sleeve to the member, with the sleeve having internal dimensions greater than the external dimensions of the member; (b) supporting the sleeve so that it is spaced from the member whereby to provide a continuous interspace from the member to the sleeve; (c) sealing the continuous interspace at its lower end; (d) filling the continuous interspace with an expansive filler; and (e) allowing the filler to set, or cure, and expand within the confines of the continuous interspace to generate an expansive pressure of at least 3 MPa which imparts at least radial prestress forces on the member and tensile hoop stresses in the sleeve, whereby the generated forces effect a transition of loading forces from one section to another of the load bearing member through the sleeve and expansive filler.
 2. A method according to claim 1 including the step of securing two or more parts together to form the sleeve about the member.
 3. A method according to claim 1 wherein the interspace is filled from its upper end.
 4. A method according to claim 1 including the step of providing spacers to evenly space the sleeve from the member.
 5. A method according to claim 1 including the steps of providing at least one toroidal bladder attached to the lower end of the sleeve with a filler tube extending to the upper end of the sleeve, and filling the bladder with expansive filler through the filler tube before filling the interspace.
 6. A method according to claim 1 including the step of sealing the interspace at its upper end.
 7. A method according to claim 1 wherein the step of filling the interspace with an expansive filler comprises filling the space with a cementitious grout containing an expansive agent of a calcium-sulpho-aluminate structure.
 8. A method according to claim 1 including the step of providing shear keys on either or both the member and the sleeve to key into the filler.
 9. A method according to claim 1 wherein the step of filling the interspace with an expansive material results in direct contact between the filler and the member and between the filler and the sleeve.
 10. Apparatus for increasing the structural strength of a load bearing member including a rigid, structural sleeve formed of one, two or more elongate sections to be fastened about a load bearing member, a plurality of spacers adapted to space the sleeve from the member to form a continuous interspace from the member to the sleeve, a seal around the lower end of the continuous interspace, and a set or cured expansive filler to fill the continuous interspace which generates and imparts at least radial prestress compressive forces of at least 3 MPa on the member and the sleeve such as to set up compressive hoop stresses in the member and tensile hoop stresses in the sleeve, the filler forming a rigid interface between the member and the sleeve.
 11. Apparatus according to claim 10 wherein the sleeve is formed of two parts adapted to be connected together.
 12. Apparatus according to claim 10 wherein said seal comprises at least one flexible toroidal bladder attached to the lower end of the respective sleeve part and adapted to be filled with said filler, the bladder having a filler line or tube extending from an upper end of the respective sleeve part.
 13. Apparatus according to claim 10 wherein said sleeve is formed of two parts hinged together along one longitudinal edge and adapted to be fastened together along another longitudinal edge.
 14. Apparatus according to claim 10 wherein the expansive filler is a cementitious grout containing an expansive agent of a calcium-sulpho-aluminate structure.
 15. A method of repairing a pile having a damaged area including the steps of: (a) securing a sleeve which has inner dimensions greater than outer dimensions of the pile around the pile, the sleeve having an axial length to, and being secured so as to, extend beyond the area of damage in both directions; (b) providing spacers to space the sleeve from the pile to form a continuous interspace from the pile to the sleeve; (c) sealing a lower end of the continuous interspace; (d) introducing an expansive filler through the upper end of the continuous interspace, displacing any water or air in the continuous interspace, to thereby fill the interspace with the expansive filler; (e) permitting the filler to set, or cure, and to expand within the confines of the continuous interspace to generate a compressive pressure of at least 3 MPa and impart at least radial prestress forces which tension the sleeve and apply compressive forces to the pile and produce compressive hoop stresses in the pile and tensile hoop stresses in the sleeve, whereby the filler facilitates load transfer primarily by friction across the filler-to-pile and filler-to-sleeve interfaces.
 16. A method according to claim 15 wherein the expansive filler is a cementitous grout containing an expansive agent of a calcium-sulpho-aluminate structure or a lime based agent.
 17. A method according to claim 15 wherein the lower end of the interspace is sealed by at least one flexible bladder attached to the lower end of the sleeve and having a filler line or tube extending from an upper end of the respective sleeve, the bladder being filled with the expansive filler or grout so as to expand and seal against the pile and retain said filler in the interspace.
 18. A method according to claim 15 wherein the lower end of the interspace is sealed by a plurality of pocket seals attached to the sleeve at its lower end, the pockets seals being open at their upper end to receive the filler introduced into the interspace, the filler filling the pockets causing inner edges thereof to engage and seal against the pile. 